Eradicating polio may not be as easy as first thought, experts have warned.

According to New Scientist magazine, the same vaccine that is being used to eradicate the disease may help to ensure the survival of the killer virus.

Polio vaccine has greatly reduced the spread of the disease

The disease used to kill or paralyse hundreds of thousands of people every year.

But since widespread vaccination began in 1955, the virus has been wiped out everywhere except for a few regions of Asia and Africa.

The World Health Organization (WHO) plans to eradicate polio by 2005.

Dangerous type

However, some experts believe the vaccine itself could pose a threat even after the disease has been wiped out.

The main vaccine being used to eradicate polio consists of a live, weakened form of the virus.

This vaccine virus could persist after the disease-causing polio virus is gone - and occasionally revert to the dangerous type.

If that happens after vaccination stops, unvaccinated children would be defenceless.

Polio experts raised these concerns at a meeting in Geneva last month.

Tjeerd Kimman, of the National Institute of Public Health and the Environment in the Netherlands, warned against stopping vaccinating once the disease had been eradicated.

Children vulnerable

The polio virus, which is spread by food and water contaminated with faeces, does not cause serious symptoms in most people, but if it spreads to the nervous system it can be deadly.

Children are particularly vulnerable to the virus.

In 1988, when the WHO launched its global eradication campaign, there were some 35,000 cases of polio worldwide.

But, ten years later, with 82 per cent of the world's children vaccinated, this number had dropped to just 3,200.

By far the most common polio vaccine in the world is oral polio vaccine (OPV), a suspension of the weakened form of the polio virus.

OPV is cheaper than alternative vaccines, easier to administer and has the advantage of inducing immunity in the gut, where the virus first invades.

Because it is 'live', OPV viruses also spread in faeces, immunising unvaccinated people who come into contact with vaccinated children.

But OPV viruses differ from their nastier relatives by only a few mutations.

Dangerous forms

Paul Fine, of the London School of Hygiene and Tropical Medicine, said conditions in the gut favour their reversion to dangerous forms.

OPV viruses shed in faeces are genetically closer to the dangerous forms, and it is well documented that viruses shed in this way can cause disease.

It is also known that OPV viruses can persist for years in people with impaired immune systems, where they also mutate faster than in healthier populations.

This could means the millions of HIV-infected people worldwide might provide a haven for these viruses.

Once vaccination has stopped, Dr Fine estimates OPV viruses might have to persist only for three to ten years before a large enough population of non-immunised people accumulated to start spreading them.

OPV makes it harder to determine when wild polio virus has been eradicated.

It may be possible to eradicate OPV viruses by continuing to vaccinate for several years after polio appears to have been eradicated using older vaccines made up of dead viruses.

However, this would be extremely expensive, and could pose a danger if the vaccine components escaped into the general population before they have been processed.